Hollow Mesoporous Carbon Spheres for High Performance Symmetrical and Aqueous Zinc-Ion Hybrid Supercapacitor

Iodine-Doped Hollow Carbon Nanocages without Templates Strategy for Boosting Zinc-Ion Storage by Nucleophilicity

Zn-ion hybrid supercapacitors (ZHCs) combining merits of battery-type and capacitive electrodes are considered to be a prospective candidate in energy storage systems. Tailor-made carbon cathodes with high zincophilicity and abundant physi/chemisorption sites are critical but it remains a great challenge to achieve both features by a sustainable means. Herein, a hydrogen-bonding interaction-guided self-assembly strategy is presented to prepare iodine-doped carbon nanocages without templates for boosting zinc-ion storage by nucleophilicity. The biomass ellagic acid contains extensional hydroxy and acyloxy groups with electron-donating ability, which interact with melamine and ammonium iodide to form organic supermolecules. The organic supermolecules further self-assemble into a nanocage-like structure with cavities under hydrothermal processes via hydrogen-bonding and π-π stacking. The carbon nanocages as ZHCs cathodes enable the high approachability of zincophilic sites and low ion migration resistance resulting from the interconnected conductive network and nanoscale architecture. The experimental analyses and theoretical simulations reveal the pivotal role of iodine dopants. The I5-/I3- doping anions in carbon cathodes have a nucleophilicity to preferentially adsorb the Zn2+ cation by the formation of C+-I5--Zn2+ and C+-I3--Zn2+. Of these, the C+-I3- shows stronger bonding with Zn2+ than C+-I5-. As a result, the iodine-doped carbon nanocages produced via this template-free strategy deliver a high capacity of 134.2 mAh/g at 1 A/g and a maximum energy and power density of 114.1 Wh/kg and 42.5 kW/kg.

Sulfate template induced S/O doped carbon nanosheets enabling rich physi/chemi-sorption sites for high-performance zinc ion hybrid capacitors

Zinc ion hybrid capacitors (ZIHCs) are encouraging energy storage devices for large-scale applications. Nevertheless, the electrochemical performance of ZIHCs is often limited by the cathode materials which show low energy density and rate capability practically. One of the efficient strategies to overcome these challenges is the development of advanced carbon cathode materials with abundant physi/chemisorption sites. Herein, we develop a sulfate template strategy to prepare sulfur and oxygen doped carbon nanosheets (SOCNs) as a potential cathode active material for ZIHCs. The as-prepared SOCNs exhibit porous architectures with a large surface area of 1877 m2 g-1, substantial structural defects, and high heteroatom-doped contents (O: 7.9 at%, S: 0.7 at%). These exceptional features are vital to enhancing Zn ion storage. Consequently, the SOCN cathode shows a high capacity of 151 mAh g-1 at 0.1 A g-1, high cycle stability with 83% capacity retention at 5 A g-1 after 4000 cycles, and a superior energy density of 103.1 Wh kg-1. We also investigate the dynamic adsorption/desorption behaviors of Zn ions and anions of the ZIHCs carbon electrodes during the process of charge and discharge by ex-situ experiments. This work highlights the significance of the integration with a large specific surface area and bountiful heteroatoms in carbon electrodes for achieving high-performance ZIHCs.

Cross-linking and self-assembly synthesis of tannin-based carbon frameworks cathode for Zn-ion hybrid supercapacitors

Carbon frameworks with well-developed porosity present broad application prospects in energy-related materials, and green preparation still face challenges. Herein, the tannins-derived framework-like carbon material is obtained by cross-linking and self-assembly strategy.The phenolic hydroxyl and quinones in tannin cross-linking react with the amine groups in the methenamine by simple stirring, which drives the self-assembly of tannins and methenamine,contributing to the reaction products being precipitated in solution as aggregates with framework-like structure. The porosity and micromorphology of framework-like structures are further enriched by the thermal stability difference between tannin and methenamine. The methenamine of framework-like structures is entirely removed by the sublimation and decomposition and the tannin is transformed into carbon materials inheriting framework-like structures after the carbonization, which offers the path for rapid electron transport. The framework-like structure, excellent specific surface area and nitrogen doping give the assembled Zn-ion hybrid supercapacitors a superior specific capacitance of 165.3 mAh·g^-1 (350.4 F·g^-1). This device could be charged to 1.87 V to power the bulb by using solar panels. This study proves that the tannin-derived framework-like carbon is a promising electrode of the Zn-ion hybrid supercapacitors, which is beneficial for value-added and industrial supercapacitors application of green feedstocks.

Status and Opportunities of Zinc Ion Hybrid Capacitors: Focus on Carbon Materials, Current Collectors, and Separators

Zinc ion hybrid capacitors (ZIHCs), which integrate the features of the high power of supercapacitors and the high energy of zinc ion batteries, are promising competitors in future electrochemical energy storage applications. Carbon-based materials are deemed the competitive candidates for cathodes of ZIHC due to their cost-effectiveness, high electronic conductivity, chemical inertness, controllable surface states, and tunable pore architectures. In recent years, great research efforts have been devoted to further improving the energy density and cycling stability of ZIHCs. Reasonable modification and optimization of carbon-based materials offer a remedy for these challenges. In this review, the structural design, and electrochemical properties of carbon-based cathode materials with different dimensions, as well as the selection of compatible, robust current collectors and separators for ZIHCs are discussed. The challenges and prospects of ZIHCs are showcased to guide the innovative development of carbon-based cathode materials and the development of novel ZIHCs.

Graphene Film with a Controllable Microstructure for Efficient Electrochemical Energy Storage

The agglomeration of graphene sheets and undesired pore size distribution usually lead to unsatisfactory electrochemical properties of reduced graphene oxide (RGO) film electrodes. Herein, crumpled exfoliated graphene (EG) sheets are adopted as the microstructure-regulating agent to tune the morphology and micro-/mesopore amounts with the aim of increasing active surface sites and ion transportation paths in electrodes. With the optimum ratio between EG and GO, the resulting 75%-EG/RGO shows significantly improved specific gravimetric capacitance (C_s) and rate capability when compared with pure RGO electrodes in a symmetrical supercapacitor system. Moreover, when coupling the 75%-EG/RGO cathode with a Zn anode to form a Zn ion hybrid supercapacitor (ZHS), the 75%-EG/RGO exhibits a much higher C_s of 327.39 F g^-1 at 0.1 A g^-1 and can maintain 91.7% capacitance after 8000 cycles. Systematic ex situ X-ray diffraction (XRD) and X-ray photoelectron spectra (XPS) measurements reveal that the charge storage mechanism is based on both reversible physical adsorption and dual ion uptake. Furthermore, the quasi-solid-state flexible ZHS also presents high capacitive performance and can maintain ∼100% capacitance under various bending states, demonstrating potential application in wearable electronics. This strategy opens up a new path for constructing high-performance graphene film electrodes.

Enhancing cation storage performance of layered double hydroxides by increasing the interlayer distance

Layered double hydroxides (LDH) can be transformed from alkaline supercapacitor material into metal-cation storage cathode working in neutral electrolytes through electrochemical activation. However, the rate performance for storing large cations is restricted by the small interlayer distance of LDH. Herein, the interlayer distance of NiCo-LDH is expanded by replacing the interlayer nitrate ions with 1,4-benzenedicarboxylic anions (BDC), leading to the enhanced rate performance for storing large cations (Na⁺, Mg²⁺, and Zn²⁺), whereas almost the unchanged one for storing small-radius Li⁺ ions. The improved rate performance of the BDC-pillared LDH (LDH-BDC) stems from the reduced charge-transfer and Warburg resistances during charge/discharge due to the increased interlayer distance, as revealed by in situ electrochemical impedance spectra. The asymmetric zinc-ion supercapacitor assembled with LDH-BDC and activated carbon presents high energy density and cycling stability. This study demonstrates an effective strategy to improve the large cation storage performance of LDH electrodes by increasing the interlayer distance.

Towards High-Energy and Anti-Self-Discharge Zn-Ion Hybrid Supercapacitors with New Understanding of the Electrochemistry

Aqueous Zn-ion hybrid supercapacitors (ZHSs) are increasingly being studied as a novel electrochemical energy storage system with prominent electrochemical performance, high safety and low cost. Herein, high-energy and anti-self-discharge ZHSs are realized based on the fibrous carbon cathodes with hierarchically porous surface and O/N heteroatom functional groups. Hierarchically porous surface of the fabricated free-standing fibrous carbon cathodes not only provides abundant active sites for divalent ion storage, but also optimizes ion transport kinetics. Consequently, the cathodes show a high gravimetric capacity of 156 mAh g^-1, superior rate capability (79 mAh g^-1 with a very short charge/discharge time of 14 s) and exceptional cycling stability. Meanwhile, hierarchical pore structure and suitable surface functional groups of the cathodes endow ZHSs with a high energy density of 127 Wh kg^-1, a high power density of 15.3 kW kg^-1 and good anti-self-discharge performance. Mechanism investigation reveals that ZHS electrochemistry involves cation adsorption/desorption and Zn₄SO₄(OH)₆·5H₂O formation/dissolution at low voltage and anion adsorption/desorption at high voltage on carbon cathodes. The roles of these reactions in energy storage of ZHSs are elucidated. This work not only paves a way for high-performance cathode materials of ZHSs, but also provides a deeper understanding of ZHS electrochemistry.

Nanostructure Nickel-Based Selenides as Cathode Materials for Hybrid Battery-Supercapacitors

Supercapacitors (SCs) have attracted many attentions and already became part of some high-power derived devices such as Tesla's electric cars because of their higher power density. Among all types of electrical energy storage devices, battery-supercapacitors are the most promising for superior performance characteristics, including short charging time, high power density, safety, easy fabrication procedures, and long operational life. An SC usually consists of two foremost components, namely electrode materials, and electrolyte. The selection of appropriate electrode materials with rational nanostructured designs have resulted in improved electrochemical properties for high performance and has reduced the cost of SCs. In this review, we mainly spotlight the nickel-based selenides nanostructured which applied as high-performance cathode materials for SCs. Different nickel-based selenides materials are highlighted in various categories, such as nickel-cobalt-based bimetallic chalcogenides and nickel-M based selenides. Also, we mentioned material modification for this material type. Finally, the designing strategy and future improvements on nickel-based selenides materials for the application of SCs are also discussed.